The present invention relates to an exhaust gas purifier equipped with a catalyst converter for purifying harmful exhaust gas discharged from engines and specifically to a diagnostic technique for the deterioration of catalyst thereof.
With intensifying regulation of harmful components (e.g., HC, CO, NOx) in exhaust gas discharged from automobile engines or the like, various diagnostic techniques for the deterioration of catalyst used for the purification of exhaust gas have been proposed.
Representative of catalytic converters is a three-way catalyst for simultaneously processing the reduction of NOx and the oxidation of HC and CO. Further, in order to enhance the purification of exhaust gas, proposed is a catalytic converter in which not only the main catalytic converter but also the pre-catalytic converter is provided.
As conventional representative diagnostic techniques for catalytic converter there are known methods, with exhaust sensors, e.g., O.sub.2 sensors, provided upstream and downstream of a catalytic converter installed in an exhaust pipe, for example, as described in Japanese Patent Laid-Open No. 5-248227 for diagnosing the deterioration of a catalytic converter from the correlation between the upstream and the downstream O.sub.2 sensors, such as e.g., the measured value of time from the output inversion of the upstream O.sub.2 sensor to the output inversion of the downstream O.sub.2 sensor when inverting the air fuel ratio from lean to rich or from rich to lean, and the output ratio, response ratio or phase ratio of the upstream and the downstream sensors.
With attention paid to the capability of oxygen storage in possession of a catalyst, every of these methods performs a deterioration judgment of the catalyst by quantitatively determining the capability of oxygen storage. With higher purification rate of exhaust gas by a catalyst (higher oxygen storage), the correlation coefficient of the output of the downstream (post-catalytic) exhaust sensor to that of the upstream (pre-catalytic) exhaust sensor becomes low, the response is delayed and the output of the downstream exhaust sensor decreases. The deterioration judgment of the catalyst is carried out, based on such characteristics.
Besides, when the catalyst is new, the oxygen storage is saturated (100% activation of a catalyst) near 300.degree. C., whereas with an advance in heat deterioration, the capability of oxygen storage does not yet reach a state of 100% activation in the range of 300.degree. C. to 550.degree. C. and may become a state of 100% activation at a higher temperature than this range because the capability of oxygen storage gradually falls (even this case has thus far been treated as a good catalytic converter in the conventional art). Accordingly an erroneous diagnosis is prevented by determining whether the catalyst is a good one or a deteriorated one, corresponding to a temperature condition at the time of diagnosis.
Japanese Patent Laid-Open No. 7-71232 proposes a diagnostic apparatus for vehicle exhaust gas purifier with a plurality of catalyst bodies arranged in a series from the upstream of an exhaust passage of an engine, in which exhaust gas sensors are provided upstream of an upstream most catalyst boy, among a plurality of catalyst bodies, and a downstream most catalyst body. When predetermined diagnostic conditions (e.g., the engine running state lies in the diagnosis range established by the engine load and the number of engine revolution, the warming-up is completed, or the vehicle is running at a constant speed) is carried out, the purification rate of a specific catalyst body is obtained from the total purification rate of the exhaust gas purifier and from the respective purification rates for the catalyst bodies except the specific catalyst body evaluated from the respective detection signals from the respective exhaust gas sensors and the deterioration condition of the catalytic converter is judged by comparing the purification rate of the specific catalyst body with a reference value.
Incidentally, other diagnostic techniques of a catalyst with attention paid to the capability of oxygen storage are described in Japanese Patent Laid-Open No. 5-98945, Japanese Patent Laid-Open No. 5-98946, Japanese Patent Laid-Open No. 5-98947, Japanese Patent Laid-Open No. 5-98948, Japanese Patent Laid-Open No. 5-98949, Japanese Patent Laid-Open No. 5-106493, Japanese Patent Laid-Open No. 5-106494, Japanese Patent Laid-Open No. 5-163989, Japanese Patent Laid-Open No. 5-180043, Japanese Patent Laid-Open No. 6-173661 and so on.
The conventional diagnostic techniques for the deterioration of catalytic converters for exhaust gas purification intend to judge the deterioration from the purification rate after the activation of a catalyst, i.e., the quantitative determination of the saturated oxygen storage capability.
Meanwhile, from the viewpoint of environmental protection, the requirements for regulating the exhaust gas become recently stringent. In order to overcome such stringent requirements, proposed is a catalytic converter in which in addition to a main catalytic converter, an early-activation catalytic converter is provided to accelerate the activation as well as a pre-catalytic converter is provided to secure the purification efficiency after the activation.
The early-activation catalytic converter basically employs a three-way catalytic converter, like as the main catalytic converter and the pre-catalytic converter. However, in order to shorten a time period to reach the activation temperature from the engine start, the early-activation catalytic converter may be provided with an electric heater, may include an accelerating catalyst for oxidation reaction, or may have a small heat capacity and be placed closer to the engine. The temperature of the early-activation catalytic converter reaches the activation temperature thereof before the temperatures of the main catalytic converter and the pre-catalytic converter reach the activation temperature. Accordingly, the early-activation catalytic converter can perform the exhaust gas purification short time after the engine start.
Under such circumstances, it is also required to diagnose the early-activation function of a catalytic converter (in particular, early-activation catalytic converter), but the existent diagnostic technique for catalysts cannot cope with this requirement. This is because the conventional diagnostic technique for catalysts diagnoses the deterioration of a catalyst from the exhaust gas purification rate after the catalyst's temperature reaches the activation temperature, and includes no technique for monitoring how much time has passed since the engine start till the catalyst is activated.
The present invention is made in view of these points and its first purpose is to provide an engine exhaust gas purifier provided with an early-activation catalytic converter in addition to the main catalytic converter, which purifier is adapted to a stringent regulating for exhaust gas.
The second purpose of the present invention is to enable the symptoms of a pre-catalytic and an early-activation converters to be distinguished and identified separately in deterioration diagnosis when the pre-catalytic and the early-activation catalytic converters are provided as auxiliary catalytic converters at the preceding step for the main catalytic converter.